1. Field of the Invention
This invention relates to a reflectometry and a reflectometer for using low coherence light to measure a reflection intensity (distribution) in a measured optical circuit such as a optical waveguides or a optical module.
2. Description of the Related Art
FIG. 1 shows an example of a low-coherence reflectometer in a related art, here a low-coherence reflectometer based on a Michelson interferometer of optical fiber type. In the figure, numeral 1 denotes a light source made of a light-emitting diode for emitting low-coherence light having a polarization degree of 0.1, a spectral band width of 50 nm, and a center wavelength of 1.53 xcexcn, numeral 2 denotes an optical fiber coupler having two input ports 2-1 and 2-2 and two output ports 2-3 and 2-4, numeral 3 denotes a measured optical module comprising an optical fiber pigtail 3a, numeral 4 denotes a polarization controller, numeral 5 denotes an optical fiber delay line made of an optical fiber coil, numeral 6 denotes a reflection mirror, numeral 7 denotes a linear stage, numeral 8 denotes a photodetector, numeral 9 denotes a signal processing system, numerals 10, 11, and 12 denote optical connectors, and numerals 13 and 14 denote collimating lenses.
In the described configuration, light emitted from the light source 1 is incident on the optical fiber coupler 2 through the input port 2-1 and is made to branch to the output ports 2-3 and 2-4. The light made to branch to the output port 2-3 is incident on the measured optical module 3 through the optical fiber pigtail 3a connected by the optical connector 10 as measurement light. The measurement light is reflected at each point responsive to the propagation distance of the measured optical module 3 and the reflected light signal propagates through the optical fiber pigtail 3a in the opposite direction and is incident on the output port 2-3.
On the other hand, the light made to branch to the output port 2-4 of the optical fiber coupler 2 passes through the polarization controller 4 and the optical fiber delay line 5, is made a collimated beam through the collimating lens 13, is reflected on the total reflection mirror 6, propagates through the path in the opposite direction, is incident on the output port 2-4 of the optical fiber coupler 2, and is used as local light signal.
Here, the optical fiber delay line 5 is provided for balancing the lengths of both arms of the Michelson interferometer of optical fiber type in response to the length of the optical fiber pigtail 3a connected to the measured optical module 3 and is replaced by means of the optical connectors 11 and 12 at both ends of the optical fiber delay line 5 whenever necessary.
The reflected light signal from the measured optical module 3 and the local light signal reflected on the total reflection mirror 6 are combined by the optical fiber coupler 2 and the mixed lightwave signal is emitted from the input port 2-2 and is made a collimated beam through the collimating lens 14, then the collimated beam is received at the photodetector 8. The beat signals of the reflected light signal and the local light signal received at the photodetector 8 and subjected to photoelectric conversion are processed by the signal processing system 9 and the reflection intensity of the measured optical module 3 is measured from the length of the signal.
In the reflectometer, the input port 2-1 of the optical fiber coupler 2 is connected to the light source 1, forming the light branch section, the input port 2-2 of the optical fiber coupler 2 is connected through the collimating lens 14 to the photodetector 8, forming the lightwave combining section, the output port 2-3 of the optical fiber coupler 2 forms the optical measurement block, and the output port 2-4 of the optical fiber coupler 2, the optical fiber delay line 5, the collimating lens 13, and the total reflection mirror 6 (containing the linear stage 7), forming the variable optical delay circuit; they make up the interferometer.
The coherence length of the emitted light from the light source 1 is about 40 xcexcm. Thus, for the reflected light signal to be able to interfere with the local light signal with respect to a specific position of the total reflection mirror 6, the light path length of the reflected light signal needs to match the optical path length of the local light signal within the coherence length. Thus, if the total reflection mirror 6 is moved in the direction of light beams on the linear stage 7, only the interference beat signals of the reflected light signal at the points of the measured optical module 3 corresponding to the total reflection mirror positions in a one-to-one correspondence can be provided and the intensity of each beat signal is measured and is multiplied by an appropriate constant, whereby the light power of the reflected light signal can be found. The spatial resolution of the reflectometry is given as xcexac/nxcex4xcexd where xcexa is a constant, c is a light velocity, n is a group index of measurement optical waveguide, and xcex4xcexd is the full width at half maximum of spectrum of emitted light from a light source. If the spectrum of the emitted light from the light source is of Gauss type, xcexa=0.31.
Since the low-coherence reflectometer uses light interference to measure the light power of reflected light signal, the polarization controller 6 needs to be used to make the polarization state of local light signal and that of the reflected light signal to match. In many cases, the optical fiber pigtail 3a is connected to the measured optical module 3 as shown in FIG. 1. If the measured optical module 3 is connected to one arm of the interferometer, the optical fiber delay line 5 needs to be connected to the other arm of the interferometer for balancing.
The polarization state of light propagating through the optical fiber changes according to bending of the fiber or the stress state. If the waveguide itself of the measured optical module has a double refraction property, the polarization state of light reflected at the points of the waveguide varies from one point to another. Therefore, to use different measured optical modules or optical fiber delay line or measure a optical waveguide having a double refraction property, the polarization controller needs to be used to adjust the polarization states of both; however, it is indispensable to eliminate the adjustment in order to save time and labor of measurement and realize fully automatic reflection measurement.
FIG. 2 shows an example of a low-coherence reflectometer in another related art, namely, a polarization-insensitive low-coherence reflectometer capable of measuring the reflection intensity of reflected light signal independently of the polarization state of the reflected light signal (namely, the light power of the reflected light signal). In the figure, numeral 15 denotes a polarizer, numeral 16 denotes a polarization beam splitter, numeral 17 denotes a photodetector, and numeral 18 denotes a signal processing system. Parts identical with those previously described with reference to FIG. 1 are denoted by the same reference numerals in FIG. 2.
In the example of the low-coherence reflectometer in the related art, a polarized wave diversity technology is adopted wherein local light signal and reflected light signal are separated into P wave and S wave by the polarization beam splitter 16, the reflected light signal and the local light signal are made to interface with each other in their respective polarization states, and the interference intensities of the beat signals of the reflected light signal and the local light signal are detected by photodetectors 8 and 17 and the signal processing system 18, and are added together.
Let the electric field elements of the P and S waves of the reflected light signal and the local light signal separated through the polarization beam splitter 16 be (Erp, Ers) and (ELp, ELs) respectively where the subscripts r and L denote the reflected light signal and the local light signal respectively.
Considering that the electric field elements of the reflected light signal and the local light signal are in a coherent state and the same phase in their respective polarized waves (P or S waves), the amplitudes of the interference beat components become ErpELp* and ErsELs* and the intensities of the beat signals are represented as
Ip=xc2xdxc2x7|ErpELp*|2
xe2x80x83=xc2xdxc2x7|Erp|2|ELp|2xe2x80x83xe2x80x83(1.1)
Is=xc2xdxc2x7|ErsELs*|2
=xc2xdxc2x7|Ers|2|ELs|2xe2x80x83xe2x80x83(1.2)
From expressions (1.1) and (1.2), the sum of both intensities becomes
I=Ip+Is
xe2x80x83=xc2xdxc2x7(|Erp|2|ELp|2+|Ers|2|ELs|2)xe2x80x83xe2x80x83(2)
The intensities of the P and S waves of the local light signal are |ELp|2 and |ELs|2 and only if they equal
|ELp|2=|ELs|2xe2x80x83xe2x80x83(3)
I=xc2xdxc2x7(|Erp|2+|Ers|2)|ELp|2)xe2x80x83xe2x80x83(4)
That is, the sum total I becomes proportional to |Erp|2 and |Ers|2, the intensity of the reflected light signal, and the light power of the reflected light signal signal can be measured independently of the polarization state of the reflected light signal.
Since the emitted light from the light source is almost non-polarized light, the local light signal is allowed to pass through the polarizer 15 in order to polarize the local light signal. Since the emitted light from the light source is non-polarized light, even if the planes of polarization are rotated in an optical fiber delay line 5, the components of the light passing through the optical fiber delay line 5, orthogonal to each other equal unless a polarization dependent loss exists in the fiber. Thus, a half the power of collimated light can always pass through the polarizer 15. The light passing through the polarizer 15 is reflected on a total reflection mirror 6, passes through the polarizer 15, and propagates through the optical fiber delay line 5 and the output port 2-4 in the opposite direction again. Since local light signal just before it is separated through the polarization beam splitter 16 generally is put into elliptical polarization, it is necessary to adjust a polarization controller 4 so as to satisfy expression (3).
Because of the described configuration, if the optical fiber delay line 5 is installed on the side of the local light signal to balance the interferometer in response to the length of an optical fiber pigtail 3a connected to a measured optical module 3 and the polarization state of the local light signal is adjusted by the polarization controller 4, a light power distribution of the reflected light signal from the measured optical module 3, namely, a reflection distribution can be measured.
From the viewpoint of automating reflection measurement, the low-coherence reflectometer in FIG. 2 also has the following disadvantages:
It becomes necessary to adjust the polarization state of the local light signal by the polarization controller 4 each time the optical fiber delay line 5 connected to the side of the local light signal is replaced. The polarization state of the local light signal varies with different optical fiber delay lines. Thus, to automate the adjustment, it is necessary to provide the polarization controller with a mechanism capable of automatically rotating a half wavelength plate and a quarter wavelength plate at any desired angle and install a system for monitoring the distribution ratio of the polarization beam splitter with respect to each rotation angle of both the wavelength plates.
In the embodiments of the invention described with reference to FIG. 4 and later, to provide high sensitivity, a differential circuit is used to reduce intensity noise which is generated when detected of the low-coherence light; however, the differential circuit cannot be built in the low-coherence reflectometer shown in FIG. 2 for separating light through the polarization beam splitter.
From the described background, development of a low-coherence reflectometer which does not require a function for arbitrarily adjusting the polarization state if the optical fiber delay line is replaced, is released from polarization adjustment, and enables a differential circuit to be built in to provide high sensitivity is demanded strongly.
It is an object of the invention to provide a polarization independent reflectometry and a polarization independent reflectometer for providing a polarization adjustment free function capable of measuring a reflection intensity (distribution) of a measured optical circuit without adjusting the polarization state of an optical fiber delay line composed on the local light signal side and not requiring any polarization adjustment.
To solve the problems, the invention is characterized by the fact that a light source for emitting low-coherence light in an almost no polarization state is used, that emitted light from the light source is separated into two parts, one is entered in a measured optical circuit as measurement light, and the other is given a group delay that can be changed arbitrarily as local light signal, then the local light signal is combined with reflected light signal provided by reflecting the measurement light at each point responsive to the propagation distance of the measured optical circuit and the reflected light signal and the local light signal are caused to interfere with each other, and that a polarizer and a polarization rotation device for arbitrarily rotating the polarization state of propagation light 0 degrees and 90 degrees at the time are composed on either the path of the local light signal arriving at the lightwave combining section or the path of the reflected light signal arriving at the lightwave combining section, whereby the sum of the intensities of the interference beat signals at polarization rotation angles is found, making it possible to measure the light power of the reflected light signal from the measured optical circuit regardless of the state of the measured optical circuit or optical fiber delay line.
In the invention, a mechanism for rotating the polarization state 0 degrees and 90 is also required. However, electric or mechanical control determined for always providing only two states of 0 degrees and 90 degrees is only required whatever the measured optical circuit and optical fiber delay line are. It is not necessary to find the optimum state such that output reaches the maximum or that two components become equal while fining adjusting a polarization controller. For example, to a Faraday rotation element used with a polarization rotation device of the invention described with reference to FIG. 5, two determined electric currents may always be allowed to flow into a coil and intensities I0 and I90 of the interference beat signals measured for the current values may be added.